`elementAttributeNameCase: "html"` is not applied to `appendChild` root properties or `setProperty` names (nested children convert)

[gtritchie]

What's the problem?

With elementAttributeNameCase: "html", the react→html attribute-name conversion is applied inconsistently depending on how a hast plugin sets the property:

• Nested descendants of a subtree passed to ctx.appendChild are converted (strokeWidth → stroke-width). ✅
• The root node passed to ctx.appendChild is not converted — strokeWidth is emitted verbatim. ❌
• ctx.setProperty names are not converted either. ❌

The same hast property name produces different JSX attribute names depending on its position, so plugin-added SVG presentation attributes leak invalid camelCase names into the rendered HTML (browsers ignore them). The root/setProperty path does apply a smaller special-case set (className → class, ariaHidden → aria-hidden), which makes the gap easy to miss until an SVG attribute is involved.

Repro (node repro.mjs):

import { mdxToJs, defineHastPlugin } from 'satteri';

const plugin = defineHastPlugin({
  name: 'repro',
  element: {
    filter: ['p'],
    visit(node, ctx) {
      ctx.appendChild(node, {
        type: 'element',
        tagName: 'svg',
        properties: { strokeWidth: 'ROOT' },
        children: [
          { type: 'element', tagName: 'path', properties: { strokeWidth: 'NESTED' }, children: [] },
        ],
      });
      ctx.setProperty(node, 'strokeWidth', 'SETPROP');
    },
  },
});

const { code } = await mdxToJs('hello', {
  hastPlugins: [plugin],
  elementAttributeNameCase: 'html',
});
console.log(code);

Output (satteri 0.9.5 — identical on 0.9.1):

    return _jsxs(_components.p, {
        strokeWidth: "SETPROP",
        children: ["hello", _jsx(_components.svg, {
            strokeWidth: "ROOT",
            children: _jsx(_components.path, { "stroke-width": "NESTED" })
        })]
    });

One property name, three spellings-in-flight, two survive to output incorrectly.

Downstream context: found while investigating why withastro/astro#17514 (which passes elementAttributeNameCase: "html" on the Sätteri MDX path, fixing withastro/astro#17512) appeared to work in Astro's test fixture but not in a real site. The fixture's attributes sat on a nested <path> (converted); the real-world plugin set them on the appended root node and via setProperty (leaked).

Environment Information

• satteri npm v0.9.5 (reproduced identically on v0.9.1), prebuilt binary @bruits/satteri-darwin-arm64
• macOS 26.6 (arm64)
• Rust: n/a (prebuilt NAPI binary)
• Node.js v24.18.0

What's the expected result?

All three positions emit "stroke-width". Property-name conversion should depend only on the configured elementAttributeNameCase, not on whether the property arrived on an appended subtree's root, a nested child, or via ctx.setProperty.